1. Field of the Invention
The present invention relates to a metallization process for manufacturing semiconductor devices. More particularly, the invention relates to a method for depositing aluminum nitride films.
2. Background of the Related Art
Surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices are circuit devices which conduct signal processing by converting electrical signals to acoustic waves, and from acoustic waves to electrical signals. The ability to transfer sound waves into electrical data, and electrical data into sound waves, have made SAW and BAW devices widely used as filters, resonators, delay lines, and other electric-acoustic devices in communications and related fields. In particular, SAW and BAW devices are highly desirable for use in the mobile radio communications field since the acoustic wave devices have a simple construction which provides effective radio filtration characteristics for use in mobile communication electronics. Recently, an increasing need has arisen for SAW and BAW devices to operate at higher frequencies to keep pace with the increasing demands for faster information transfer in the mobile radio communications field.
As an example, a SAW device is manufactured by forming electrodes of a conducting film, e.g., a metal, called inter-digital transducers (IDTs) on a piezoelectric substrate, with the electrodes interconverting the electronic signals and the surface acoustic waves. The characteristics of surface acoustic waves depend upon the propagation characteristics of the electrodes on the piezoelectric substrates. For acoustic wave devices to operate at higher frequencies, the piezoelectric substrates and the electrodes must have high propagation velocities for the acoustic waves.
Piezoelectric materials having desirable electromechanical properties, such as high propagation velocities, high frequency response, and high coupling coefficients, may be attained by using composites of thin piezoelectric films with materials used as bulk wave propagation media, surface wave propagation substrates and mechanical resonators. Therefore, one preferred method of achieving high frequency operations of acoustic wave devices is to deposit a high-speed acoustic wave propagation film on a piezoelectric film or substrate.
Aluminum nitride (AlN) films have been considered as an attractive piezoelectric material for fabrication of on-chip high frequency GHz-band acoustic wave devices (greater than 2 Ghz) because of aluminum nitride's attractive electromechanical properties which include a high propagation velocity of about 6000 m/s, a high coupling coefficient of about 0.07 which represents the efficiency of conversion of electrical energy to acoustic wave energy, good piezoelectricity properties, and high thermal stability. These electromechanical properties are dependent on the film's physical parameters which include the crystal structure, the crystal orientation, and the thickness of the film. By optimizing the film parameters, devices with desired electromechanical properties can be produced for specific uses in SAW applications. Therefore, significant effort has been undertaken to produce aluminum nitride films of high crystalline quality, preferred grain orientation (often referred to as crystal orientation), and uniform thickness for use in SAW applications, and in particular, for use in the fabrication of on-chip acoustic wave devices for integrated circuits.
Conventionally, thin aluminum nitride (AlN) films are deposited by chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) techniques. While these AlN films showed very promising acoustic wave properties when evaluated by surface acoustic wave techniques, these deposition methods for AlN films typically require a high substrate temperature in excess of 1000.degree. C. This high deposition temperature is not compatible with current semiconductor integrated circuit manufacturing methods for the fabrication of on-chip acoustic wave devices.
One alternative technique for depositing AlN films at temperatures lower than 500.degree. C. is to deposit the AlN film by a PVD sputtering method. However, aluminum nitride layers deposited by PVD methods tend to produce layers having random crystal orientations. The aluminum nitride crystal structure which forms randomly on a substrate produces polycrystalline films without a preferred grain orientation. A random crystal orientation leads to a degradation in conversion between electronic signals and surface acoustic waves by producing films having low coupling coefficients, and impaired acoustic propagation velocities with higher propagation losses. Thus, the orientation of the crystal in a polycrystalline film is a key material parameter affecting the SAW and BAW characteristics of deposited films, and the crystal orientation formation in the AlN film is important to the formation of a film having superior acoustic properties.
Thin film deposition processes typically begin with the deposition of isolated nucleation sites. In a PVD deposition, these sites are created when individual sputtered portions of the target hit the substrate, followed by other such portions depositing on the substrate. In a CVD deposition, the nucleation sites occur where the precursor deposits on the substrate. It is believed that the difference in the growth rates of the nucleation sites determines the film's orientation. In particular, it is believed that films deposited by PVD sputtering produce polycrystalline films originating and growing from random nucleation sites. Nucleation sites having crystal orientations which exhibit fast growth rates, such as those with a growth direction normal to the substrate surface will emerge as predominant over sites having orientations exhibiting slower growth rates. The resulting film structure from the PVD sputtering process has multiple grains of varying sizes and multiple crystal orientations.
One proposed method of producing an aluminum nitride film by PVD with a preferred degree of crystal orientation is disclosed in Ishihara et al., "Control of Preferential Orientation of AlN Films Prepared by the Reactive Sputtering Method", Thin Solid Films 316, pp. 152-157 (1998). Ishihara et al. provide a sputtering process regime for depositing aluminum nitride films having a preferred crystal orientation. However, the Ishihara et al. process produces films that are highly stressed and have non-uniform film thickness. The high film stress produces layering defects in the film that have a detrimental affect on the electromechanical properties of the aluminum nitride films. Further, since the transmission frequency and frequency response of a SAW device is very dependent on the uniform thickness of a film, non-uniform films provide variable and inconsistent SAW performance. Additionally, the Ishihara et al, process suffers from low deposition rates of less than 200 angstroms per minute in comparison to commercially acceptable deposition rates of about 1000 angstroms per minute or greater, thereby limiting the commercial application of the process.
Therefore, there remains a need for a method for depositing aluminum nitride films that provides a preferred crystal orientation and improved film properties with high deposition rates.